Ed,
You should try eBay. I think you may be able to get more than $500 for these, if the condition is good.
I used to have a KC500 about four years ago and I agree that they are really heavy. I thought about buying two KC550's but I decided to get the 350's because of the weight and I thought the sound was good enough, I can always add a sub for bottom end. The price for two 550 was also about the same as two 350 + a sub. The newer KC350 and KC550 have much better sound and features, like stereo link, sub woofer out etc ...
[This message has been edited by harosha (edited 03-23-2005).]